Tooth type and toothless Permanent Magnet (PM) motors produce torque between the stator and the PM rotor via the application of appropriate electric currents through two or more stator phase windings. Rotational torque is generated in the tooth type motor (where windings are tucked into slots, or behind tooth tips fashioned in the stator iron) through directed magnetic attraction and repulsion between the rotor magnets and the stator teeth.
Toothless or slotless motors (where the winding is in the flux gap) produce torque by bending the PM rotor flux field as current is applied to the stator windings. Tooth type motors typically exhibit high electrical inductance due to the smaller magnetic gap and the higher volume of iron (the coils are wrapped about iron teeth) in close proximity to the winding. Higher values of inductance decrease motor controllability due to restricted actuator response bandwidth. Tooth type motors and Electromagnetic (EM) bearings tend to be non-linear in torque and force production for high values of torque, and can completely fail to produce additional force or torque on peak demands due to flux saturation in the tooth type iron structures. Tooth-type motors, whether rotating or positioning exhibit far higher torque ripple or cogging than toothless motors due to the strong magnetic attraction or saliencies between stator teeth and rotor magnets.
Internal forces loading the motor shaft can be caused through differential magnetic pull created by deviations in rotor magnet-to-magnet strength as well as rotor centrality errors. These large forces increase exponentially as the rotor moves further off-center, positioning some of the rotor magnets closer to the stator iron. Tooth type motors with permanent magnet rotors exhibit a strong pull to a preferred axial position due to the sharp decrease in magnetic circuit reluctance as rotor magnets and back iron become axially aligned. Toothless motors have far less axial pull due to the inherently larger magnetic gap that accommodates the conductors positioned within the magnetic flux gap.
In toothless motors, axial forces are present between individual rotor magnets and adjacent current carrying coil end-turns, but the geometric balance over the entire motor results in the gross cancellation of most of these forces. Normally the rotor shaft must be supported in both the axial and radial directions by a bearing system, which allows the shaft to freely turn despite variations in the external shaft load, which includes the forces due to the working load, belt tension, reaction forces in gears and the internal rotor to stator forces described above. The typical motor shaft is supported via two (2) rolling element mechanical radial bearings, one on each end of the shaft, although other arrangements are sometimes used. A thrust bearing is typically integrated into the function of one or more of the mechanical radial bearings. The types of bearings often found in BLDC motors include: mechanical rolling element (ball or roller bearings), solid lubricated sleeves/thrust plates, fluid film (gas or liquid) types such as hydrostatic and hydrodynamic bearings.
Electromagnetic (EM) bearings are used, when cost is less of a factor, to maintain the shaft rotational centerline and thrust position without mechanical contact. Current EM motor bearing system designs include a BLDC motor combined with discrete Maxwell (tooth type) bearings and gap or position sensors. The typical resulting combination of EM Bearing and BLDC motor is physically large due to the lack of a common housing and magnetic circuit integration. In addition, the resulting system is significantly susceptible to loading by the forces described above, and suffers performance debits due to the high inductance. Due to the large magnetic gap and sufficiently thick radial band of back iron, toothless motors are quite immune to magnetic saturation, however; the removal of heat generated within the winding throughout such peak demands requires significant design consideration. Toothless motors have a hollow cylindrical volume for conductors traversing the length of the flux gap, allowing numerous choices for the placement and configuration of the conductors which form coils that form the stator.
The winding configurations used in existing toothless motors include the use of individual “single pole” pre-wound coils, pre-formed coils, or folded and inserted coils (sometimes called “skein” wound coils), that are subsequently positioned onto the stator assembly, and aligned with virtual slots. Complete motor structures can be wound in numerous configurations on a fixture where conductors are forced into virtual slots by tooling thus producing the same effect as individual wound coils described above. These configurations are all planar about a cylinder in the sense that the coils are distributed equally about a centerline, and that the mid-point of the coils are centered on a plane normal to and located on the centerline. FIG. 1 illustrates the placement of a single phase into virtual slots as a wave winding.
FIG. 1 depicts a BLDC Motor Typical Conductor Configuration (one phase of a wave winding is shown) with centerline and mid-plane. The back iron is not shown in FIG. 1, but can be placed on the outside of the coil for an inside rotor motor, or placed inside the coil when constructing an outside rotor motor. The rotor construction incorporates iron; often called “Return Iron” backing the permanent magnets. FIG. 2 depicts a line drawing of a self-standing cylindrical zigzag 3-phase motor winding and illustrates a self-supporting 3-phase Zigzag winding configured for use in a radial gap motor. The application of this winding to inside or outside rotor motors and the construction of suitable permanent magnet rotors and back iron are details familiar to those skilled in the art.
An axial gap motor (not shown), places the PM rotor between two back-iron or return path plates with pie shaped coils positioned in the flux gap on one or both sides of the rotor, and has motoring attributes similar to those described above.